The object of the invention is a transmitter/receiver for transmitting and receiving an RF signal in two operating frequency bands.
Mobile station systems have developed and expanded at an extremely rapid rate, which is the reason why a variety of systems using many different standards have been or are being constructed in many areas. This has generated the need for mobile stations which can use more than one system. As an example one could mention the digital GSM system and DCS that is PCN system which operate in different frequency bands but whose specifications are otherwise similar to each other.
From the published patent application EP 653851, a transmitter/receiver arrangement is known in which one local oscillator is used and its frequency has been selected between the lower operating frequency band and the higher operating frequency band in such a way that the same intermediate frequency can be used when operating in both operating frequency bands. The weak point of this solution is, however, that due to the need for these intermediate frequency stages, the implementation is extremely complicated and due to the great amount of components it requires, the manufacturing costs of such a device are high.
In a direct conversion receiver, that is in a zero intermediate frequency receiver, a radio frequency signal is converted directly to a baseband without there being any intermediate frequency. Since no intermediate frequency stages are needed, only a few components are needed in the receiver, which makes it a preferable solution for various applications. However, in mobile stations, direct conversion receivers have so far rarely been used.
FIG. 1 shows a prior known schematic block diagram of a transmitter/receiver of a mobile station and in this block diagram the receiver is a so-called direct conversion receiver. An RF signal received by an antenna 138 is conducted via a duplex filter 102 to a pre-amplifier 104. The purpose of the duplex filter is to permit the use of the same antenna both in transmitting and receiving. Instead of a duplex filter, also a synchronous antenna changeover switch can be used in a time-division system. The RF signal which is received from the amplifier 104 is low-pass pass or band pass filtered 106 and demodulated in an I/Q demodulator 108 into an in-phase signal 108a and into a quadrature signal 108b. A local oscillator signal 114b which is needed in the demodulation is received from a synthesizer 114. In block 110, removal of dc-voltage as well as automatic gain control AGC are carried out. Block 110 is controlled by a processing block 116 which may contain, for example, a microprocessor and/or a digital signal processor DSP. Automatic gain control is regulated by a signal 110a and removal of the offset voltage is regulated by a signal 110b. Signals received from block 110 are converted into digital signals in block 112 from which the signals are further transferred to digital signal processing circuits in the processing block 116.
The transmitter unit comprises an I/Q modulator 128. This takes an in-phase signal 128a and a quadrature signal 128b and creates a carrier frequency signal which is low-pass filtered and/or high-pass filtered by a filter 130. The carrier frequency signal is amplified by an RF amplifier 132 and the amplified signal is transferred via a duplex filter 102 to an antenna 138. A power control unit 134 of the transmitter controls the amplification of the RF amplifier 132 on the basis of the measured output power 136 and of the control 134a received from the processor.
FIG. 1 also shows, attached to the processing unit, a memory unit 126 and user interface means which comprise a display 118, a keyboard 120, a microphone 122 and an earpiece 124.
Practical solutions for the implementation of a direct conversion receiver have been described more closely, for example, in the following publications:
1! Microwave Engineering Europe, January 1993, pages 59 . . . 63, PA1 2! Microwave Engineering Europe, May 1993, pages 53 . . . 59 and PA1 3! published patent application EP 0 594 894 AI. PA1 said receiver comprises at least one RX mixer for mixing the received signal into a baseband signal, PA1 said transmitter comprises at least one TX mixer for mixing the baseband signal into a carrier frequency transmitting signal and PA1 the transmitter/receiver comprises synthesizer means for forming the first PX mixing signal to the RX mixer for mixing the signal which has been received in the first receiving frequency band into a baseband signal and for forming the first TX mixing signal to the TX mixer for mixing the first baseband TX signal into the first carrier frequency TX signal in the first transmitting frequency band, is characterized in that the transmitter/receiver comprises additionally PA1 the first conversion means for forming the second RX mixing signal from the first signal formed by said synthesizer means for mixing the signal which has been received in the second receiving frequency band to the second baseband RX signal and PA1 the second conversion means for forming the second TX mixing signal from the second signal formed by said synthesizer means for mixing the second baseband TX signal into the second carrier frequency signal in the second transmitting frequency band.
FIG. 2 shows a solution for the implementation of a transmitter/receiver which operates in two frequency bands. An RF signal received by the antenna is connected either to the DCS branch or to the GSM branch of the circuit via a switch 204. If a DCS frequency band signal is being received, the received signal is conducted to a band pass filter 206, to a low noise amplifier LNA 208 and to a band pass filter 210. Thereafter, components which are separated by a phase shift of 90 degrees are formed from the signal in block 212. The in-phase component I and the quadrature component Q are conducted further by switches 214 and 234 to mixers 216 and 236. A mixing signal for the mixers is obtained from a DCS synthesizer 240, the frequency of which corresponds to the received carrier frequency and then an in-phase and a quadrature component of a complex baseband signal are obtained as a result of this mixing process. The baseband signal is processed further in a processing unit of a received signal, which means an RX signal, block 239.
Similarly, when a GSM signal is being received, the switch 204 controls the received signal to the GSM branch in which there are, respectively connected in series, a band pass filter 226, a low noise amplifier 228, a band pass filter 230 and a phase shifter 232 which forms two signals which are separated by a phase difference of 90 degrees. The signals are conducted further, controlled by the switches 214 and 234, to the mixers 216 and 236 in which a signal selected by a switch 261 and obtained from a GSM synthesizer 250 is now used as mixing frequency. Signals obtained from the mixers are conducted further to the processing unit 239 of a baseband received signal, which means an RX signal.
The DCS synthesizer is formed, as known, from a phase locked loop PLL which comprises a voltage controlled oscillator VCO 241, the output signal of which is amplified by an amplifier 246 for forming an output signal. The frequency of a signal transmitted by the oscillator 241 is divided by an integer Y in a divider 242 and the resulting signal is conducted to a phase comparator 243. Similarly, the frequency of the signal formed by a reference oscillator 258 is divided by an integer X in a divider 244 and conducted to the phase comparator 243. The phase comparator produces a signal which is proportional to the phase difference of said two input signals and which has been conducted to a low pass filter LPF 245, and the filtered signal controls further the voltage controlled oscillator 241. The above described phase locked loop operates in a known manner so that the output frequency of the synthesizer becomes locked to the frequency which is led to the phase comparator from the reference frequency branch. The output frequency is controlled by changing the dividing number Y.
The GSM synthesizer 250 comprises respectively a voltage controlled oscillator 250, an amplifier 256, dividers 252 and 254, a phase comparator 253 and a low pass filter 255. The GSM synthesizer operates in a similar way as the above described DCS synthesizer but the output frequency of the GSM synthesizer corresponds to GSM frequency bands.
In the transmitting unit, the baseband complex transmitting signal, which means the TX signal, is processed in the processing unit of a TX signal and from there the in-phase and the quadrature component of the signal are conducted to mixers 262 and 282 in which a carrier frequency signal is formed by multiplying the input signal by the mixing signal. If the DCS frequency is used in the transmission, the output signal of the DCS synthesizer is selected via a switch 261 as a mixing signal. The carrier frequency signal is conducted via a switch 264 to the DCS branch in which a phase shift of 90 degrees is formed first between the in-phase component and the quadrature component, and after this, the received signals are summed, block 266. The formed DCS signal is conducted to a band pass filter 268, to an amplifier 270 and to a band pass filter 272. The formed RF signal is conducted further to an antenna 202 via a switch 280.
If the transmission takes place in the GSM frequency band, the output signal of the GSM synthesizer is used as the mixing signal. The received carrier frequency signal is conducted to the GSM branch in which a similar processing occurs as in the DCS branch in blocks 286, 288, 290 and 292. The formed RF signal is conducted to the antenna 202 via the switch 280. To permit the use of the same antenna 202 both in transmitting and in receiving, the transmitting and the receiving circuits have to be connected to the antenna, for example, via a Duplex filter as in the arrangement shown in FIG. 1. When operating in two frequency bands, filters are needed for each frequency band. Instead of the Duplex filter, also a synchronized antenna changeover switch can be used in a time-division system.
One disadvantage of the above described circuit arrangement is that it requires the use of two synthesizers, which increases considerably the complexity and the manufacturing costs of the transmitter/receiver.
Another problem connected to the above presented solution is achieving an adequate phase accuracy. The accuracy demand for the phase difference between the I and the Q components is only of a few degrees' magnitude. Since in conventional RC phase shifters, factors on which the phase shift depends include the frequency and the temperature of the components, it is difficult to achieve an adequate phase accuracy throughout the entire frequency band and in all operating conditions. In addition, operating in two frequency bands which are far from each other complicates the controlling of the phase accuracy.
One solution is to form signals in different phases of a higher oscillator frequency by dividing the signals in which case a better phase accuracy is achieved which is independent on the frequency. The disadvantage of this solution is, however, that when operating, for example, in the 2 GHz frequency band, one would need a synthesizer with an output frequency of 4 GHz which is such a high frequency value that the implementation of the synthesizer and the frequency dividers would become extremely complicated.